1. Field of the Invention
The invention relates to a crucible for the crystallization of silicon and to the preparation and application of release coatings for crucibles used in the handling of molten materials that are solidified in the crucible and then removed as ingots, and more particularly to release coatings for crucibles used in the solidification of polycrystalline silicon.
2. Description of the Related Art
Crucibles of silica (either of fused-silica or of quartz) are typically used in solidification of polycrystalline silicon. Silica is chosen primarily for high-purity and availability. There are problems in using silica, however, as a crucible for the production of silicon by this method.
Silicon in its molten state will react with the silica crucible that is in contact with it. Molten silicon reacts with silica to form silicon monoxide and oxygen. Oxygen will contaminate the silicon. Silicon monoxide is volatile, and will react with the graphite components inside the furnace. Silicon monoxide reacts with graphite to form silicon carbide and carbon monoxide. The carbon monoxide will then react with the molten silicon, forming additional volatile silicon monoxide and carbon. Carbon will contaminate the silicon. Silicon can also react with the various impurities contained in the silica crucible (iron, boron, aluminum, . . . ).
The reaction between silica and silicon promotes adhesion of the silicon to the crucible. This adhesion, combined with a difference in coefficients of thermal expansion between the two materials, creates stress in the silicon ingot, causing it to crack on cooling. It is known in the art that a release coating applied to the inside of the crucible in the area of contact with the ingot can prevent the reaction between silicon and silica that leads to ingot contamination and cracking. To be effective, the release coating must be thick enough to prevent the silicon from reacting with the silica crucible, and must not adversely contaminate the silicon either by itself or from contaminants within it.
A variety of materials and techniques are described in the literature, which attempt to solve the problem of reaction and adhesion of the crucible in contact with molten material. For example U.S. Pat. No. 5,431,869 describes a multi-component release agent of silicon nitride and calcium chloride for silicon processing using a graphite crucible. This document teaches a crucible for the crystallization of silicon in which the inner crucible wall is coated with a silicon nitride powder to form a first layer having a thickness from 150 to 300 micron. This document does not teach the formation of other layers to produce the crucible as defined in present claim 1.
U.S. Pat. No. 4,741,925 describes a silicon nitride coating for crucibles applied by chemical vapor deposition at 1250° C. while WO-A1-2004053207 discloses a silicon nitride coating applied by plasma spraying. U.S. Pat. No. 3,746,569 discloses the pyrolysis formation of a silicon nitride coating on the walls of a quartz tube. U.S. Pat. No. 4,218,418 describes a technique of forming a glass layer inside a silica crucible by rapid heating to prevent cracking of silicon during melt-processing. U.S. Pat. No. 3,660,075 discloses a coating of niobium carbide or yttrium oxide on a graphite crucible for melting fissile materials. The niobium carbide is applied by chemical vapor deposition, while the yttrium oxide is applied as a colloidal suspension in an aqueous inorganic solution.
Prior art references include specific references to powdered mold release agents for application to crucibles in the directional solidification of silicon. In addition, the use of chemical vapor deposition, solvent evaporation, high-temperature flame treatment, and other expensive and complex means are mentioned for application of crucible coatings. References are made to specific binders and solvents. References are made to mixing, spraying, or brushing for slurries of powdered coatings.
This silicon nitride release coating itself can lead to problems. The thickness of the silicon nitride coating necessary to prevent the silicon from reacting with the silica crucible is quite important (about 300 μm) making thereby the coating operation expensive and time consuming. Further, this silicon nitride coating is mechanically weak and can peel or flake off during or even before use. It is therefore recommended to apply this coating at the very last moment before use, i.e., at the end user facilities, leaving thereby the burden of applying this thick coating to the end user.
In the co-pending International application WO-A1-2005106084, the applicant has suggested to use a crucible for the crystallization of silicon comprising a base body comprising a bottom surface and side walls defining an inner volume; an intermediate layer comprising 50 to 100 wt. % of silica at the surface of the side walls facing the inner volume; and a surface layer comprising 50 to 100 wt. % of silicon nitride, up to 50 wt. % of silicon dioxide and up to 20 wt. % of silicon on the top of the intermediate layer.
Although this crucible represents already a significant step forward with respect to the prior art, there remains some room for improvement. In particular, if during the crystallization of the silicon ingot, for any reason, the silicon ingot adheres to the surface layer, cracks will be generated at the surface of and will spread through the ingot during cooling down.
The co-pending International application WO-P01-2005106084 suggests to overcome this problem by limiting the adhesion of the intermediate layer to the base body and suggests, to this end, to act on the porosity of the intermediate layer. The present invention has for its objective to propose an alternative solution achieving this result.